1. Bug report:
When I cast
Metallic Mimic as the first creature, the list doesn't contain Saproling.
If I cast it when there is a Saproling on the battlefield, the list does contain Saproling.
I guess the list is implemented by taking all the creature cards sub-types from my library and from the creatures on the battlefield, and therefore it behaves like that.
In order to support Saproling it will have to process the text of non-creature cards which may be very hard/impossible - am I right?
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2. General suggestion regarding impossible card list
An impossible card may has several effects, and only some of them are impossible.
The card may still be useful even when it works partially.
For example I would like to use some of the cards with Aftermath while ignoring the Aftermath part.
Maybe implementing the impossible cards partially with a message of the impossible reason on the card text is a good solution?